This invention relates to an integrated high voltage generating system, and more particularly to an integrated high voltage generating system having a charge pump.
Various products have recently been developed which are portable and compact, and, as a result, they tend to have a lower supply voltage obtained from a single power source. For example, the power voltage of portable radio or portable tape recorder is about 1.5 to 3.0 V. When a high power voltage is required for a product, a high voltage generating system is incorporated in the product or a semiconductor device used in the product. When an integrated high voltage generating sytem is incorported in a semiconductor device, the high voltage is generated by using a charge pump. Furthermore, if a high voltage power source having different potentials is needed, a high voltage is first generated by the charge pump, and this high voltage is divided and stepped down. The output is controlled using this divided voltage, thereby creating another high voltage source different from the original high voltage source. However, since the current supply capacity of the charge pump in the device is smaller in relation to its area, the electric power consumed in this dividing circuit cannot be ignored.
A conventional integrated high voltage generating system using a charge pump is described below.
FIG. 7 shows a conventional example of an integrated high voltage generating system using a charge pump for delivering two different power voltages. It comprises a charge pump having four unit circuits connected in series composed of diode-connected N-channel MOS transistors 1 to 5 and capacitors 6 to 9, a dividing circuit composed of two MOS transistors 17, 18, and a step-down control part composed of a MOS transistor 11 gated by the divided voltage of the dividing circuit.
The operation of the conventional integrated high voltage generating system is explained by referring to FIG. 7.
The electric charge coming in through the MOS transistor 1 from power source V.sub.cc connected to the power source input terminal is sequentially transferred among four stages of unit circuit controlled by clock signals CLK, CLK of normal and reverse phases with an amplitude of V.sub.cc, and is sequentially boosted in this process. The boosted power supply voltage is delivered to a first power supply output terminal OUT 1. Part of this power supply voltage is delivered to a second power supply output terminal OUT 2 as another power supply voltage, by way of a step-down MOS transistor 11 which is gated by the divided output of the dividing circuit composed of two MOS transistors 17, 18. However, when a dividing circuit is used within the integrated high voltage generating system, the stability and response of this circuit and the current consumption are in conflict with each other, and accordingly the current consumption cannot be significantly decreased. The reason is as follows. Usually, at the first power supply output terminal OUT 1, a current flows of about several microamperes. On the other hand, when the MOS transistors 17, 18 are fabricated according to the design rule of about 3 microns, the gate width (W) and gate length (L) of the gate electrode are usually about 60 microns and 3.0 microns respectively and their ratio W/L is about 20. At this time, the circuit IL flowing in the MOS transistors 17, 18 is about 5 mA. That is, when such MOS transistors 17, 18 are used, the majority of the current of several milliamperes delivered from the charge pump is consumed in the dividing circuit, and the function as the integrated high voltage generating system is sacrificed. Hence, as one of the methods of decreasing the current consumption in the dividing circuit to, for example about 5 .mu.A, it may be considered to design the gate width (W) at about 3 microns and the gate length (L) about 1500 microns, that is, the W/L of about 0.002. In turn, hoever, when the gate of MOS transistors 17, 18 is fabricated in such a size, since the gate width (W) itself is only about 3 microns, if its dimension varies about .+-.0.3 micron, the effect of fluctuation is significant, and the stability of the device is impaired. Or when the gate is reduced to such a size, the stray capacitance of the gate increases. As a result, the response of the circuit is worsened. Thus, in the conventional integrated high voltage generating system using a dividing circuit, it was extremely difficult to lower the current consumption without spoiling the stability and response of the circuit.
Additionally, as another method of decreasing the current consumption, it may be considered to raise the dividing resistance. For example, when dividing 20 V into 15 V, in a 3-micron process, by using a MOS transistor of about 3 mm.sup.2, the current consumption becomes about 100 .mu.A. To reduce this current consumption, additional MOS transistors as the dividing resistances should be inserted between the power supply terminals of the dividing circuit (more than a dozen in the above example). Thus, the value of the current flowing in the dividing circuit may be reduced to sevaral microamperes, and the current consumption can be decreased. This method however, becomes sensitive to fluctuations of the threshold voltages of the MOS transistors. Accordingly the circuit stability is poor.
On the other hand, the charge pump power supply is extremely small in the supply electric power per umit area, as compared with the power supplied by merely dividing the external power source. Therefore, if the capacity of the charge pump is increased in order to compensate for the slight power consumption in the dividing circuit, the area occupied by the charge pump within the semiconductor device becomes extremely large, and the chip size itself becomes larger.
It is hence a first object of this invention to present an integrated high voltage generating system capable of obtaining a potential between the input power supply voltage and the output power supply voltage without consuming unnecessary current.
It is a second object of this invention to present an integrated high voltage generating system capable of obtaining a potential between the input power supply potential and the output power supply potential without causing the chip area to be increased.
This invention may be briefly summerized as an integrated high voltage generating system including a charge pump having a plurality of unit circuits composed of diodes or diode-connected MOS transistors and capacitors of which one end is connected to the input side of the diodes or the MOS transistors and the other end to the clock signal source, connected in series so that the diode polarities may be in a same direction and clock phases applied to adjacent capacitors may be mutually reverse, wherein an intermediate potential is taken out from the intermediate node between the power supply input terminal of the charge pump and the first power supply output terminal, and this intermediate potential is applied to the gate of the MOS transistor connected between the first power supply output terminal and the second power supply output terminal.
In this constitution, without consuming the unnecessary current as experienced in the dividing circuit, and without causing the chip area to be increased, a potential between the input power supply potential and the first power supply output potential may be obtained from the second power supply output terminal.
Other features and objects of the present invention will be apparent from the following description taken in connection with the accompanying drawings.